1. Field of the Invention
The present invention relates to a charged particle beam exposure method used in the lithography process of semiconductor integration circuits and the associated mask. In particular, the present invention relates to a charged particle beam exposure method employing an electron beam and an ion beam according to the divided pattern transfer method to improve the accuracy of stitching of the patterns.
2. Description of the Related Art
The exposure method employing the charged particle beam will be explained as related art. The current charged particle beam exposure method has a high accuracy but a low throughput. To solve this problem, a variety of techniques have been developed including a partial pattern projection exposure method e.g. a cell projection, a character projection and a block exposure which is utilized at present. According to the partial pattern projection exposure method, a small pattern, e.g., squares of 5 microns, is projected as an exposure unit. If a small repetitive pattern of the circuit require exposure, such as squares of 5 micron, then each small pattern is exposed one after another using a mask on which different kinds of small repetitive patterns are formed. However, if some of the patterns are not repetitive, they are exposed according to the variable shaped beam exposure method.
A new charged particle beam reduction projection device is proposed according to the charged particle beam exposure method that enables a much higher throughput than that of the conventional pattern projection exposure method. This is possible due to the following process: a mask is prepared on which the entire pattern of a semiconductor chip is formed; following this the charged particle beam is irradiated onto a given area of the mask, and the image of a pattern laid within the area is transferred in reduction by a projection lens.
However, irradiating the charged particle beam simultaneously to the whole area of the mask does not enable accurate transfer of the pattern, due to aberrations. In addition, it is difficult to prepare the original plate of the mask. As a result of these problems, a method has recently been studied where a die (a chip on a wafer) or a plurality of dies are not exposed simultaneously; instead, small areas divided from the pattern (we call "subfield") are exposed using an optical system with an optical field which does not, however, cover the whole chip. This method will be referred to as the divided pattern transfer method. The exposure is carried out by adjusting the focus of the image of a subfield to be projected onto the exposure surface thus minimizing aberrations such as distortion of the subfield for each individual subfield. This enables an exposure to have an excellent resolution and accuracy within a large area compared with a simultaneous transfer of the entire die.
Using the charged particle beam exposure method, the areas that can be simultaneously exposed, called the unit exposure areas, are smaller than that of the optical exposure method employing visible or ultraviolet light. Accordingly, since it is difficult to simultaneously expose the entire pattern of a semiconductor chip, it is necessary to stitch exposed portions divided from the pattern. The stitching might be accompanied by a gap or displacement, which may be reduced by improving the accuracy of pattern transfer.
Meanwhile, the mask, which is the original plate used for the wafer exposure, is also prepared by the charged particle beam exposure. To fabricate the mask, adjusting, e.g., moving and overlapping, the boundary of the pattern is possible.
This method improves the accuracy of the stitching, but reduces the throughput owing to the repeated exposures necessary. The reduction in the throughput does not affect the mask production seriously, but it affects the wafer production. Accordingly, this method is not available for the wafer exposure.